1. Field of the Invention
This invention relates to the precise attachment of a first member to a second member, and provides a method of and apparatus for reforming the second member by a variable magnitude depending on the actual position of the second member relative to a predefined net reference position. The invention relates, in particular, to the precise attachment of an outer body panel member of an automotive vehicle to an imprecisely located element of an inner body panel structure.
2. Description of the Related Art
In the automotive industry, the assembly of vehicles involves the attachment of outer body panels to inner body panels or body structures. It is therefore important that the inner body panel attachment positions be precisely located with respect to predetermined three-dimensional coordinates so that variations thereof are not translated through the attachment of the outer body panels so as to be visible to the customer.
W. S. Zaydel, et al., U.S. Pat. No. 4,438,971, describes a method of and apparatus for attaching, at a precise location, a plastic automotive body panel to a relatively imprecisely located element of an inner body panel structure. According to the teachings of this patent, plastic-filled metal attachment blocks or pads are welded at selected locations to the inner body panel structure. The inner body panel structure is then positioned within a locating and machining fixture and each of the plastic-filled attachment blocks is milled to a precise position, the distance between the inner body panel structure and the milled face of the plastic-filled attachment blocks varying with the position of the inner body panel structure. The plastic outer body panel member is then attached to the milled face of the plastic-filled attachment block, after drilling a hole in the plastic-filled attachment block to receive a threaded fastener or a fastener attached to the inside of the plastic outer body panel member. By this method and apparatus, each such plastic outer body panel member is mounted in a precisely determined and in precise surface alignment with each adjacent plastic panel.
The method and apparatus of the aforesaid U.S. Pat. No. 4,438,971 does lead to precision in the positioning of plastic outer body panels to a relatively imprecisely positioned automotive inner body panel structure, but it requires the attachment of the plastic-filled attachment blocks to the inner body panel structure which adds to the material and processing costs. Additionally, the subsequent milling of the plastic-filled attachment blocks generates scrap which is a mixture of a metal and the plastic filler material which, for the disposition of such scrap in an environmentally acceptable manner, involves additional expense and, in any case, the milling of the plastic-filled attachment blocks generates dust corresponding, mainly, to the composition of the plastic filler. This is also objectionable on environmental and workplace health and safety grounds, unless strict precautions are followed in performing the milling and drilling of the plastic-filled attachment blocks.
It is also known in the prior art that a relatively imprecisely located automotive inner body panel element can be built up to provide for the attachment thereto of an outer body panel element at a more precisely determined location by attaching one or more metal shims to the surface of the inner body panel element to which the outer body element is to be attached, the number of such shims to be attached based on the original position of the inner body panel element. However, this is a time-consuming and expensive procedure, and the attachment of such shims adds to the weight of the vehicle, an undesirable feature especially since it detracts from fuel economy.
Dacey, Jr., U.S. Pat. Nos. 4,760,633 and 4,884,431 disclose a method and apparatus for body panel attachment which reforms a portion of an inner panel member to a precise location. This method and apparatus employs an anvil that is moved into a known position defining the desired precise location of the inner panel attachment surface. Floating support members engage and thereby locate the inner panel, and are then locked in a fixed position.
A hydraulic ram is actuated and moves a pad forming member mounted thereon under force against the inner panel to deform a portion of the inner panel between the floating support members into engagement with the anvil. This results in an attachment pad being formed in the inner panel whose surface is located at the precise location desired for attachment to another member, such as an outer body panel. While the pad forming member is still in engagement with the anvil, a punching operation is performed to form an aperture in the pad. A punch is moved by a separate hydraulic cylinder through the pad to form the aperture.
While Dacey, Jr. does provide an alternate method for precisely assembling an outer body panel to an imprecisely located inner body panel, both patents rely on hydraulic actuation of the ram and punch. Hydraulic equipment generally suffers from the disadvantages of significantly increased cost and maintenance requirements, slow operation, and auxiliary cooling requirements. Additionally, hydraulic devices include a pressurized fluid, typically oil, that occasionally leaks as a result of long term use within the environment of an assembly operation. Hydraulic leaks are messy, carcinogenic, environmentally unacceptable, and present a serious fire hazard. Finally, the heating and consequent expansion of the hydraulic fluid must be taken into consideration for precise control of variable speed and/or variable stroke hydraulic ram devices.
Jackson, U.S. Pat. No. 5,133,206 discloses a method and apparatus for forming a portion of a panel member to a predetermined reference position. The apparatus includes a sensing device that locates the actual position of a panel member. Next, an extendible and retractable rod positions a holding means including a first and second pair of arms at the actual located position of the panel member established by the sensing device. A drive means is then actuated to move the first and second pair of arms into engagement with the panel member to hold adjacent sides of the portion of the panel member to be reformed to the predetermined reference position.
After locating and holding the panel member, a forming means including a fourth pair of arms is moved to urge a forming member into the panel member to reform the portion of the panel member to the predetermined reference position. An aperture is optionally pierced in the reformed portion of the panel member by sequentially urging a piercing pin mounted on the end of a third pair of arms into the reformed portion of the panel member.
The invention disclosed by Jackson also includes the steps of camming the first and second pair of arms between the first and second positions, as well as camming the third and fourth pair of arms from a first position spaced from the panel member to a second position such that the forming member and piercing pin engage the panel member. The step of camming the holding means from the first to the second positions is initiated before the step of camming the forming means is initiated so as to bring the holding means into engagement with the panel member before the forming means engages the panel member.
The Jackson invention identifies the same problem addressed by Dacey, Jr., and teaches an apparatus adapted to provide the same solution taught by Dacey, Jr. Although Jackson alleges an improved apparatus, the only support provided for such allegation is that the apparatus employs a single actuation device for holding the inner body panel in a fixed position. The Jackson invention, however, remains problematic for the following reasons.
The apparatus taught by Jackson is excessively complex and thus unnecessarily expensive and subject to failure. More specifically, Jackson incorporates numerous moving components including a positioning means having an extendible and retractable rod, a hydraulic or pneumatic drive means responsive to the output of a sensor device, and four pair of adjustable arms each having a cam device wherein each moving component gives rise to an additional mode of failure.
Furthermore, Jackson discloses a complex sensor device adapted to locate the panel member and transmit a corresponding output signal to the drive means that is controlled thereby. This sensor device is a critical feature of the disclosure whereby any imprecision thereof is proportionally translated to the attachment pad formed on the inner body panel, and potentially gives rise to a visibly misaligned outer body panel. As the effectiveness of the Jackson apparatus is predicated on the sensor device maintaining a high degree of precision over numerous cycles, the device becomes increasingly expensive. Finally, Jackson does not disclose a device for analyzing the precision of the sensor device for purposes of error proofing, whereby loss of precision thereof will likely result in numerous defectively assembled automobiles before it can be identified and addressed.
Copeman, U.S. Pat. No. 5,440,912 discloses an apparatus adapted to form a workpiece to a net position and pierce a hole in the workpiece. The apparatus generally includes an inner and an outer slide assembly. The outer slide assembly includes a hydraulic form cylinder slidably mounted to a base, the form cylinder has a form punch mounted thereon via a pair of outer slide rails. The inner slide assembly includes a hydraulic pierce cylinder, including a form back-up and is coupled to a form cylinder rod end via a cylinder mounting block extension.
The workpiece is positioned between the form back-up of the inner slide assembly and the form punch of the outer slide assembly. Actuation of the form cylinder compels the cylinder mounting block extension along with the entire inner slide assembly toward the workpiece until the form back-up contacts the workpiece. A slave cylinder connected to the inner slide assembly is fillable through a one way valve so that when the form back-up of the inner slide assembly contacts the workpiece, the slave cylinder locks the form back-up in place.
The form cylinder continues to push the form cylinder rod end thereby moving a form punch and an anvil, connected via the outer slide rails, toward the workpiece. The form cylinder extends a predetermined amount to emboss the workpiece to a net position. After the workpiece has been embossed, a pierce cylinder of the inner slide assembly actuates a punch to pierce a hole in the workpiece. The use of hydraulic cylinders by Copeman present the same disadvantages for this device as set forth with regard to the device of Dacey, Jr.
The inventions of Dacey, Jr., Jackson and Copeman disclose similar devices. Jackson claims to require fewer steps than Dacey, Jr., and Copeman claims to be more compact, however, the devices accomplish the same objective in much the same way. One of the common features of Dacey, Jr., Jackson and Copeman includes an actuation device having constant velocity and stroke. More sophisticated programmable punching and forming motions including variable velocity and stroke are advantageous for a number of reasons described in detail hereinafter.
Variable velocity punching and forming devices reduce cycle time by varying the speed of the ram over the cycle with a rapid ram advance and retraction combined with slower speeds as the work piece is approached as well as during actual punching or forming. Variable velocity allows for a broader working range in that a single actuation device can accommodate a variety of different size dies and/or punches, and can form and/or punch a variety of different materials and material configurations.
Variable stroke forming devices permit positive depth control of a formed feature. Variable stroke punching devices are capable of making a variety of different sized and shaped holes with a single cutting tool. This is accomplished by, for example, providing a cutting tool having an initial cutting surface and a secondary cutting surface a predetermined axial distance therefrom, wherein the effective diameter of the secondary engagement surface is necessarily larger than that of the initial engagement surface. A variable stroke device incorporating such a cutting tool can form a small hole using a short stroke such that the secondary cutting surface does not contact the material to be punched. The same device can also form a larger hole by using a longer stroke such that both the initial and secondary cutting surfaces pass through the material to be punched.
Finally, an apparatus having a programmable punching motion in combination with a linear transducer provides automatic error proofing of the process to ensure that a hole has been punched. As is well known in the art, the current signature provided by the linear transducer is representative of punch/metal resistance, as the resistance encountered by the punch greatly increases during engagement with the metal panel, a current spike is generated. Accordingly, a current spike indicates a successful punching operation, and conversely the absence of a current spike indicates the desired punching operation was not performed (i.e. the punch broke and did not engage the panel).
From the above, it can be appreciated that the prior art methods and apparatuses for precisely positioning and attaching a first member to an imprecisely located second member by forming a portion of the second member to a predetermined net reference position are not fully optimized. Therefore, what is needed is a punching and forming apparatus having variable velocity and stroke, which does not rely on hydraulic actuation, that is simple, reliable and inexpensive as well as environmentally acceptable.